The present invention relates to a system for enabling surgery to be performed by vibrational heating and more particularly to a system for enabling surgery to be performed by ultrasonic heating guided by magnetic resonance (MR) imaging.
Conventional Magnetic Resonance Imaging (MRI) provides the radiologist with internal views of a subject's anatomy. MRI provides excellent contrast between different tissues and is useful in planning surgical procedures. A tumor in a subject is much more visible in an MR image than as seen in actual surgery because the tumor and normal tissue often look similar in surgery. The tumor can also be obscured by blood during surgery. Researchers at Brigham and Womens Hospital, Boston, Mass. have proposed treatment of deep lying tumors by laser surgery. F. A. Jolesz, A. R. Bleire, P. Jakob, P. W. Ruenzel, K. Huttl, G. J. Jako, "MR Imaging of Laser-Tissue Interactions", Radiology 168:249 (1989). Thus, in the case of brain tumors, the subject is first scanned in an MRI system to locate the tumor and plan a safe trajectory between the entry and target points. This can be accomplished by a MRI device employing fast scan apparatus such as U.S. Pat. Nos. 4,961,054 Gradient Current Speed-up Circuit for High-speed NMR Imaging System by John N. Park, Otward M. Mueller, and Peter B. Roemer, issued Oct. 2, 1990, or 5,017,871 Gradient Current Speed-up Circuit for High-speed NMR Imaging System, by Otward M. Mueller, and Peter B. Roemer, issued May 21, 1991 both assigned to the present assignee and hereby incorporated by reference. A view of the heated region is provided with the use of MR temperature sensitive pulse sequences. Known MR temperature sensitive pulse sequences are described in U.S. Pat. No. 4,914,608 In-vivo Method for Determining and Imaging Temperature of an Object/Subject from Diffusion Coefficients Obtained by Nuclear Magnetic Resonance, Denis LeBihan, Jose Delannoy, and Roland L. Levin issued Apr. 3, 1990. Experiments on animals show that a heated zone above a critical temperature destroys tissue. This zone increases in size with time as the heat is applied to reach a steady state or both temperature and heat flow. If the maximum temperature is limited to 100 deg. C., then the heated zone, the area exceeding a critical temperature causing destruction of tissue, approaches 1 centimeter in diameter. It is difficult to predict the heated zone geometry because the heat flow depends on the profusion of blood as well as the tissue thermal properties.
Tumors have been selectively destroyed in cancer subjects using focused ultrasound heating at the University of Arizona, as reported by B. E. Billard, K. Hynynen and Robert. B. Roemer Effects of Physical Parameters on High Temperature Ultrasound Hyperthermia Ultrasound in Med. & Biol. Vol. 16, No. 4, pp. 409-420, 1990 and hereby incorporated by reference. Billard et al. disclose that the control of heat is improved by using short heating pulses where the effect of blood perfusion is negligible. However, since they do not image the temperature distribution, it is difficult to hit small, deep laying targets.
It would be beneficial to be able to accurately localize heat to selectively kill or destroy tumor tissue without damage to surrounding healthy tissue.